The invention concerns a device for measuring the temperature near the liquid crystals of a liquid crystal display (LCD), and more specifically a device for measuring the temperature near the liquid crystals of a liquid crystal display (LCD) covered with a touch screen.
There is a growing market for electronic devices of the type having a LCD display for displaying visual information and a touch screen covering the LCD by which the user communicates with the device.
Liquid crystals are characterized by their ability to change their optical properties in response to applied electromagnetic fields. This has made them ideal for displaying altering information, such as in liquid crystal displays (LCD). This ability is affected by the temperature of the liquid crystals, which in its turn is dependent on a number of factors, such as air temperature, radiation from the sun, heat generated by electronic equipment in the vicinity of the display etc. The effects of varying LCD temperature is especially pronounced when the optical state of the liquid crystals is determined by low-voltage multiplexing technique, since this puts high requirements on the flexibility of the crystals.
It is therefore desirable to determine as close as possible the actual temperature of the liquid crystals. To obtain as good an estimation as possible of the temperature of the liquid crystals a temperature sensor should be disposed as near the liquid crystals as possible. Numerous patents are concerned with this task.
For example, the abstract of the Japanese patent JP-A-9-160001 describes a LCD control system wherein the contrast of the LCD is controlled by way of measuring the ambient temperature, and adjusting the contrast after comparing the measured temperature to a stored temperature and a contrast setting associated with the stored temperature. A similar system is also described in the abstract of the Japanese patent JP-A-8-114785. Such a control system has the disadvantage that the contrast adjustment is based on an ambient temperature that may differ considerably from temperature of the liquid crystals. This is the case regardless of whether the temperature is measured outside of the equipment in which the LCD is mounted, or inside the housing of the equipment.
It is also known, for example through JP-A 9-258161, to attach a small thermocouple sensor on a LCD display in order to measure its temperature. However, this has the disadvantage that only a point estimation of the LCD temperature is obtained. Therefore, considerable errors could occur, for example in a case where sunlight falls on only a part of the equipment.
Furthermore, through the Japanese patent publication No. 54-064998 it is known to cover a surface of a LCD display with a tin oxide film, and measuring the resistance of the tin oxide film to determine the temperature of the LCD. The tin oxide film may also act as a heating means, to provide a suitable operation temperature to the liquid crystals. If practiced in conjunction with a touch screen, this invention has the disadvantage that even a highly transparent additional temperature sensing layer would undesirably reduce the visual contrast of the LCD. Since the touch screen that covers the LCD causes a certain loss of contrast and brilliance in itself, it is generally not desirable to cover the LCD with yet another transparent sheet.